Mixed halide hybrid perovskites are of significant interest because their bandgap can be tuned as a current-matched top-cell in tandem photovoltaics. However, several mixed halide perovskites phase segregate under illumination, exhibit large voltage deficits, and produce unstable photocurrents. We investigate the origin of phase segregation and implication for tandems with mixed halide large-bandgap (∼1.75 eV) perovskites. We show explicitly that MAPb­(I0.6Br0.4)3 and (MA0.9,Cs0.1)­Pb­(I0.6,Br0.4)3, termed “MA” and “MACs”, respectively, rapidly phase segregate in the dark upon 1 sun equivalent current injection. This is direct experimental evidence that conduction band electrons or valence band holes are the culprit behind phase segregation. In contrast, (FA0.83,Cs0.17)­Pb­(I0.66,Br0.34)3, or “FACs,” prepared at only 75 °C resists phase segregation below 4 sun injection. FACs prepared at 165 °C yields larger grains and withstands higher injected carrier concentrations before phase segregation. The FACs and MACs devices sustain near constant power output at 1 sun and do not affect the current output of a CIGS bottom cell when used as an incident light filter.

Wide bandgap MAPb(IBr) perovskites show promising potential for application in tandem solar cells. However, unstable photovoltaic performance caused by phase segregation has been observed under illumination when y is above 0.2. Herein, we successfully demonstrate stabilization of the I/Br phase by partially replacing Pb with Sn and verify this stabilization with X-ray diffractometry and transient absorption spectroscopy. The resulting MAPbSn(IBr) perovskite solar cells show stable photovoltaic performance under continuous illumination. Among these cells, the one based on MAPbSn(IBr) perovskite shows the highest efficiency of 12.59% with a bandgap of 1.73 eV, which make it a promising wide bandgap candidate for application in tandem solar cells. The engineering of internal bonding environment by partial Sn substitution is believed to be the main reason for making MAPbSn(IBr) perovskite less vulnerable to phase segregation during the photostriction under illumination. Therefore, this study establishes composition engineering of the metal site as a promising strategy to impart phase stability in hybrid perovskites under illumination.

selenization temperature to obtain a 900 nm CZTSSe fi lm with larger grain size. Upon full device processing on this fi lm, a solar cell with an active area power conversion effi ciency of 8.3% was achieved without an antirefl ection coating on a cell with an area of 0.43 cm 2 . These early experiments show the importance of redox reaction in solution as well as the optimization of device fabrication conditions.The improved precursor solutions were made by adding the precursors into the solvent step-by-step as shown in Figure 1 b. First, 4 mL DMSO was added to a vial containing 4 mM Cu(OAc) 2 ·H 2 O, resulting in a deep blue suspension due to very limited solubility of Cu(OAc) 2 ·H 2 O in DMSO. Second, precursor SnCl 2 ·2H 2 O (2.75 mM) was added into the vial, and the deep blue suspension gradually changed to clear light green solution after overnight stirring. Adding the third precursor ZnCl 2 (2.80 mM) into the solution resulted in further color change from light green to lighter green, with a yellow tint. Finally, 13.2 mM thiourea was added, and a transparent and colorless solution was obtained after thiourea completely dissolved. The change from deep blue suspension to a very light green-yellow clear solution after the fi rst two steps indicates reduction of Cu 2+ to Cu + (Equation (1)) accompanied by solvation/stabilization of metal ions by DMSO and/or chloride anions. The fi nal colorless solution indicates that little Cu 2+ remains after adding thiourea. The data suggest that complexation between metal ions and thiourea form more stable Cu + species which assist the reduction of the remaining Cu 2+ to Cu + .(1)In contrast to the previous approach, in which all precursors are added simultaneously and the redox and complexation reactions compete and may result in kinetically stabilized solutions with remaining Cu 2+ , the current procedure provides a pathway for the redox reaction to complete which greatly reduces the amount of Cu 2+ . The yellow color in the previous solution (Figure 1 a) is due to unreacted Cu 2+ which form CuCl 4 2− (a yellow species) in high Cl − containing solution (note that solvated Zn 2+ , Sn 2+ , and Sn 4+ result in colorless solutions). The detailed chemistry in the precursor solution is still under investigation, and the results will be reported in follow-up work.The redox-equilibrated precursor solution (meaning that the redox-reaction has equilibrated) as reported here is very stable; no precipitation or color change has been observed after several months stored in sealed vial at room temperature. The solution was diluted by 35% with DMSO before it was spin-coated on a molybdenum coated soda-lime-glass (SLG) substrate to form the precursor fi lm. Seven coatings were applied for a total Kesterite-based thin fi lm photovoltaic cells using copper zinc tin sulfi de, selenide, and sulfoselenide, Cu 2 ZnSnS 4 (CZTS), Cu 2 ZnSnSe 4 (CZTSe), and Cu 2 ZnSn(S,Se) 4 (CZTSSe), respectively, as absorber layer are very promising for inexpensive photovoltaic electricity generation due to the ...

A unified model for the direct gap absorption coefficient (band-edge and sub-bandgap) is developed that encompasses the functional forms of the Urbach, Thomas-Fermi, screened Thomas-Fermi, and Franz-Keldysh models of sub-bandgap absorption as specific cases. We combine this model of absorption with an occupation-corrected non-equilibrium Planck law for the spontaneous emission of photons to yield a model of photoluminescence (PL) with broad applicability to band-band photoluminescence from intrinsic, heavily doped, and strongly compensated semiconductors. The utility of the model is that it is amenable to full-spectrum fitting of absolute intensity PL data and yields: (1) the quasi-Fermi level splitting, (2) the local lattice temperature, (3) the direct bandgap, (4) the functional form of the sub-bandgap absorption, and (5) the energy broadening parameter (Urbach energy, magnitude of potential fluctuations, etc.). The accuracy of the model is demonstrated by fitting the room temperature PL spectrum of GaAs. It is then applied to Cu(In,Ga)(S,Se)2 (CIGSSe) and Cu2ZnSn(S,Se)4 (CZTSSe) to reveal the nature of their tail states. For GaAs, the model fit is excellent, and fitted parameters match literature values for the bandgap (1.42 eV), functional form of the sub-bandgap states (purely Urbach in nature), and energy broadening parameter (Urbach energy of 9.4 meV). For CIGSSe and CZTSSe, the model fits yield quasi-Fermi leveling splittings that match well with the open circuit voltages measured on devices made from the same materials and bandgaps that match well with those extracted from EQE measurements on the devices. The power of the exponential decay of the absorption coefficient into the bandgap is found to be in the range of 1.2 to 1.6, suggesting that tunneling in the presence of local electrostatic potential fluctuations is a dominant factor contributing to the sub-bandgap absorption by either purely electrostatic (screened Thomas-Fermi) or a photon-assisted tunneling mechanism (Franz-Keldysh). A Gaussian distribution of bandgaps (local Eg fluctuation) is found to be inconsistent with the data. The sub-bandgap absorption of the CZTSSe absorber is found to be larger than that for CIGSSe for materials that yield roughly equivalent photovoltaic devices (8% efficient). Further, it is shown that fitting only portions of the PL spectrum (e.g., low energy for energy broadening parameter and high energy for quasi-Fermi level splitting) may lead to significant errors for materials with substantial sub-bandgap absorption and emission.